The present invention is related to a magnetic recording medium on which large capacity information can be recorded, a producing method of the medium, and a magnetic recording system. Especially, the present invention is related to a magnetic recording medium which is suitable for high density magnetic recording, and producing method of the medium, and a large capacity magnetic recording system using this magnetic recording medium.
Today, it is strongly required to increase storage capacity of a magnetic recording system. To reproduce signals from fine recorded bits, in relation to a magnetic head, a composite type head is being employed rapidly in which a magnetoresistive head (MR) having higher efficiency than a conventional inductive head is used as a reproducing element. Further, to obtain higher efficiency, a head which utilizes very large magnetoresistive effect (giant magnetoresistive effect or spin valve effect) produced in a multi-layered magnetic layer in which plural magnetic layers are piled up with insertion of a non-magnetic layer between the magnetic layers, is being used actually. On the other hand, to obtain a magnetic recording medium on which high density magnetic recording is possible, it is required that out put voltage at high linear recording density region is increased and that medium noise is decreased simultaneously. Especially, being accompanied by employing of a high efficiency magnetic head, it is strongly required for a magnetic recording medium to decrease its medium noise.
As a magnetic recording medium, a in-plane magnetic recording medium is used which comprises a Co-based alloy magnetic layer as follows: CoNiCr, CoCrTa, CoCrPt, CoCrPtTa, CoCrPtB, CoCrPtTaB, etc. Especially, a Co-based alloy magnetic layer containing Pt has high coercivity and high output voltage at high linear recording density region, therefore, it is suitable for high density magnetic recording. These Co-based alloy have hexagonal closed packed structure (h c p structure) of which c axis is easy magnetization axis, and, to use as a in-plane magnetic recording medium, it is desired that the c axis is oriented to in-plane direction. Therefore, it is a generally used method that an underlayer having body-centered cubic structure (b c c structure) is formed on a substrate first, then a Co-based alloy magnetic layer is grown with epitaxial growth mode on the underlayer, and the c axis is oriented to in-plane direction as a result.
As the underlayer, Cr is used generally, but, when a magnetic layer contains large atoms such as Pt, etc., methods to orient c axis of a magnetic layer along parallel direction to the film surface, are disclosed, in which lattice matching between a magnetic layer and an underlayer is improved by using Cr alloy underlayer of which lattice space is increased with addition of Ti to Cr (disclosed in Japanese patent publication number 63-197018) or with addition of V to Cr (disclosed in U.S. Pat. No. 4,652,499).
About materials of the underlayer besides the above mentioned materials, it is disclosed in Japanese patent publication number 63-187416 that extensive materials including Mo, W, Hf, etc. can be used. Especially, a Cr-based alloy to which Ti is added, as described in Journal of Applied Physics (J. Appl. Phys.), vol. 79, pp5351-5353, has fine crystal grains and accordingly has low medium noise, therefore it is a suitable underlayer material for high density magnetic recording. Further, it is disclosed in Japanese patent publication number 63-187416 that signal to noise ratio and reproduced signal modulation can be improved by forming an underlayer consisting of double layers. An example of the underlayer having double layers structure is disclosed in Japanese patent publication number 7-73427 and 8-30954, in which 2nd underlayer consisting of a Cr-based alloy with additive elements Mo, Zr, Ta, V, Nb, Ti, is formed on 1st underlayer comprising Cr. Further, as disclosed in Japanese patent publication number 10-74314, medium noise is decreased by providing a Co-based non-magnetic alloy layer under the above-mentioned underlayer. As disclosed in Japanese patent publication number 10-143865, medium noise is decreased even more and in a stable manner because Cr or Zr having high oxidation tendency is contained in this Co-based non-magnetic alloy layer and its surface is oxidized a little by exposing the surface in oxygen atmosphere. A magnetic recording medium in which a Cr-based underlayer having b c c structure is formed on Cr-based composite film (seed layer) containing Zr, Ti, etc., is disclosed in Japanese patent publication number 9-265619 and 10-214412.
According to the above mentioned conventional technologies, as a magnetic recording medium to be suitable for high density magnetic recording, a magnetic recording medium which comprises a Co-based alloy magnetic layer containing Pt to obtain high coercivity easily in combination with a CrTi alloy underlayer to be able to have small crystal grains, is considered. But, the magnetic recording medium having such structure has technological problems that in-plane orientation of c axis in the magnetic layer is loose and that squareness of magnetization curve or coercivity squareness is easy to decrease. If coercivity squareness s* becomes excessively small, output voltage at high linear recording density is decreased. Especially, this tendency is remarkable in the case of magnetic layers comprising CoCrPtTa alloy, CoCrPtB alloy, or CoCrPtTaB alloy to which a lot of Pt and Ta, or Pt and B are added to obtain high crystal magnetic anisotropy field required to have high coercivity.
It is 1st object of the present invention to solve the above mentioned problems and to provide a magnetic recording medium having high signal to noise ratio at high recording density region.
It is 2nd object of the present invention to provide a producing method which produces with high repeatability a magnetic recording medium having high signal to noise ratio at high recording density region.
It is 3rd object of the present invention to provide a small size large capacity magnetic recording system having high recording density over 5 Gbits per square inch (5 Gbits/in2).
To solve the above mentioned problems, the inventors of the present invention examined a lot of magnetic recording media with various structures and it was found that, by forming 2nd Cr-based alloy underlayer containing at least one element selected from a group comprising Mo and W between a magnetic layer and a Cr-based alloy underlayer containing Ti, the c axes of the magnetic layer were oriented along parallel direction to the film surface and the crystal grain diameters were decreased, namely superior results were obtained.
As a results of analyzing by X-ray diffraction a crystalline structure of a magnetic layer with addition of Pt and Ta, or Pt and B which was formed directly on a CrTi underlayer, it was confirmed that the underlayer had b c c structure and its {001}surface was oriented along parallel direction to the film surface. And, it was also confirmed that the magnetic layer formed on the underlayer had h c p structure and c axes of the magnetic layer was oriented along vertical direction to the film surface. Generally, It is well known that a magnetic layer having h c p structure with (11.0) orientation grows generally in epitaxial mode on a underlayer having b c c structure with (001) orientation as above mentioned. The above mentioned orientation is differ from this, and it is considered that the epitaxial growth is prevented by some reasons. In the case of using CrMo or CrW underlayer instead of the CrTi underlayer, it was confirmed that a magnetic layer having h c p structure with (11.0) orientation grew in epitaxial mode on a underlayer with (001) orientation, therefore, it is supposed that adding Ti caused chemical or structural variation on the surface of the underlayer, and this affected the epitaxial growth.
In the case of using CrMo or CrW underlayer, the crystal grain diameter is larger than that of the CrTi underlayer and it is difficult to maintain sufficient low noise characteristics. Against this, when CrMo or CrW layer as a 2nd underlayer was inserted between the CrTi underlayer and the magnetic layer, the 2nd underlayer with (001) orientation grew in epitaxial mode on the CrTi layer with (001) orientation, (11.0) orientation of a magnetic layer with additives of Pt and Ta, or Pt and B was achieved on this 2nd underlayer, and then in-plane orientation of the c axis could be obtained.
On the other hand the crystal grain diameter became smaller than that in the case of using CrMo or CrW single underlayer because it was determined by small crystal grain size of CrTi underlayer formed first.
As described above, in a magnetic recording medium having Co-based alloy magnetic layer containing Pt formed on a substrate through a underlayer, by forming said underlayer with plural underlayers including double layer structure which is formed by piling up 1st underlayer of Cr-based alloy containing Ti and 2nd underlayer of Cr-based alloy containing at least one element selected from a group comprising Mo and W, in this sequence, crystal grain diameters can be decreased, c axes can be oriented to in-plane direction, and then said 1st object is achieved.
It is desirable that the Ti content in said 1st underlayer is 5 at. % or more and 35 at. % or less. When the Ti content is less than 5 at. %, the effect to make crystal grain small is decreased, and when the Ti content is more than 35 at. %, the lattice matching between the underlayer and the Co-based alloy magnetic layer containing Pt which is suitable for high density magnetic recording, becomes poor and the crystalline property of magnetic layer is deteriorated. It is suitable that a film thickness of the 1st underlayer is 5 nm or more and 40 nm or less. When the thickness is less than 5 nm, it becomes difficult to control the crystalline property and crystal orientation of the layer to be formed on it, and when the thickness is more than 40 nm, efficiency of mass production is decreased, simultaneously the crystal grain diameter becomes large, the medium noise is increased, and then such thicker layer is unsuitable.
It is desirable that the sum of the Mo content and the W content in said 2nd underlayer is 5 at. % or more and 80 at. % or less. When the content is less than 5 at. % or more than 80 at. %, the lattice matching with the Co-based alloy magnetic layer containing Pt which is suitable for high density magnetic recording becomes poor and sufficient high coercivity is not obtained. The 2nd underlayer comprises an alloy consisting of Cr and at least one element selected from a group comprising Mo and W, but other elements may be added if necessary within the limited amount. It is desirable that the sum of the content of the other elements is about 2 at. % or less. But, it is desirable that Ti is not contained in the 2nd underlayer
Even when Ti is contained in the 2nd underlayer, it is desirable that the Ti content is 1 at. % or less. In higher content than this, when the Co alloy magnetic layer contains both Pt and Ta, or both Pt and B, the c axis of the magnetic layer is easy to be oriented along vertical direction to the film surface and such medium is unsuitable for a in-plane magnetic recording.
It is suitable that a thickness of the 2nd underlayer is 1 nm or more and 40 nm or less. When the thickness is less than 1 nm, the effect by which the c axis of the Co based magnetic layer containing Pt formed on the underlayer is oriented along in-plane direction, is almost lost, and when the thickness is more than 40 nm, an efficiency of mass production is decreased, and the crystal grain diameter becomes large, then the medium noise is increased, and then such thicker layer is unsuitable also. It is more suitable that the thickness of the 2nd underlayer is 1 nm or more and 20 nm or less, because higher signal to noise ratio can be obtained in the case.
As the Co based alloy magnetic layer containing Pt, following alloys are used: CoCrPt, CoCrPtTa, CoCrPtTi, CoCrPtNb, CoCrPtB, CoCrPtTaB, etc.
By adding Pt to the Co based alloy magnetic layer, it is possible to obtain a high coercivity required for a high density magnetic recording. It is desirable that the Pt content is 3 at. % or more and 25 at. % or less. When the Pt content is less than 3 at. %, the effect to increase the coercivity is small. Inversely when the Pt content is more than 25 at. % being too high, the coercivity is also decreased. It is more suitable that the Pt content is 6 at. % or more and 20 at. % or less.
It is suitable that the Pt content is kept in above range and the sum of the Mo content and the W content in said 2nd underlayer is 5 at. % or more and 80 at. % or less, because it is easy to obtain the coercivity of 200 kA/m (2.5 kOe) and it is able to form bits (magnetization transitions) of high linear recording density over 300 kFCI. Cr in these magnetic layers is segregated around a boundary of the crystal grain, accordingly this matter decreases exchange interaction between the magnetic crystal grains and decreases the medium noise. It is desirable that the Cr content is 18 at. % or more to obtain sufficient low medium noise. But, when the Cr content is more than 25 at. % excessively, saturated magnetization becomes small excessively and the coercivity is decreased, then such higher content is unsuitable. By adding also Ta with adding Pt to the magnetic layer, the higher coercivity is obtained. It is desirable that the Ta content is 1 at. % or more and 5 at. % or less. When the Ta content is lass than 1 at. %, the both effects to increase the coercivity and to promote the segregation of Cr are small, and when the Ta content is more than 5 at. %, the crystal property is deteriorated. It is suitable that the sum of the Co content and the Pt content in whole composition elements of the magnetic layer is 80 at. % or less and the coercivity measured by a vibrating sample magnetometer under a magnetic field given to in-plane direction is 200 kA/m or more, because a read/write characteristics in a high recording density region is superior. But when the coercivity is over 320 kA/m being too high, a over write characteristics is deteriorated, therefore it is suitable that the coercivity of the magnetic recording medium has controllable magnitude in a range in which the over write is possible. Furthermore, to improve wear durability, it is suitable that a C based protective layer is formed on the magnetic layer and a lubricant layer is still formed on it.
In the magnetic recording medium of the present invention, a seed layer which comprises Co based or Ni based alloys such as NiP, CoCrZr, NiCrZr, etc. consisting of substantially amorphous or fine crystalline metal film, can be formed between the substrate and the underlayer. By forming the seed layer, the medium noise can be decreased still more. This effect to decrease the medium noise by the seed layer, is especially remarkable when glass ceramics or glass with reinforced surface is used as the substrate. By using Co or Ni as main component of the seed layer, high adhesion strength to the substrate is also obtained. Further, by adding Cr as an additive element, ferromagnetic component of Co or Ni contained in the seed layer is decreased effectively, magnetization of the seed layer can be made negligible small from the view point of the reproducing head, and simultaneously high corrosion resistance is obtained. Further, by adding Zr, the seed layer can be made amorphous without deterioration of the corrosion resistance.
By means of that the surface of the seed layer CoCrZr, NiCrZr, etc. is exposed in oxygen atmosphere and oxidized a little, the crystal grain of the Cr alloy underlayer formed on it can be made small and crystal orientation of the Cr alloy can be made into (100) orientation by which the c axis of the Co alloy can be oriented along parallel direction to the film surface. Accordingly, the medium noise can be decreased stably. This effect is remarkable when CrTi is used as Cr alloy underlayer formed on the seed layer. Here, xe2x80x9cfine crystalxe2x80x9d means that a crystal grain diameter is 8 nm or less, and xe2x80x9csubstantially amorphousxe2x80x9d is defined as a structure of which diffraction pattern is observed as halo when its selected area electron diffraction image is photographed by a transmission electron microscopy. Such fine structure of the layers which constitute a magnetic recording medium, can be evaluated by a transmission electron microscopy with high magnification, or by a pattern of diffraction ring on a selected area electron diffraction image, using specimens which are-prepared in a such way that the recording medium is sliced thin in vertical direction to the substrate surface or the substrate is made thin by a mechanical polishing, and next the thin specimen is made further thinner by a ion-milling method which mills the specimen upward and downward.
The said 2nd object is achieved by employing a producing method of the above-mentioned magnetic recording medium characterized by the processes including a process to prepare a substrate, a process to heat the substrate previous to forming a 1st underlayer on the substrate, a process to form a 1st underlayer, a 2nd underlayer, and a magnetic layer in this sequence in a vacuum chamber by sequential sputtering method after heating the substrate. By heating the substrate previous to forming the 1st underlayer, impurity gas absorbed on a under surface of the 1st underlayer is eliminated and the 1st underlayer having stable crystal orientation can be formed. By forming the 2nd underlayer and the magnetic layer on this 1st underlayer sequentially without exposing in the atmosphere during the processes, stable epitaxial growth of each layer can be attained. Further, by setting the heating temperature in a suitable range, non-magnetic elements as Cr, etc. can be separated at boundary of the magnetic crystal grains, and then the medium noise can be decreased.
The said 3rd object is attained by providing a magnetic recording system comprising a magnetic recording medium, a driving mechanism which drives said magnetic recording medium, a magnetic head which comprises a recording part and a reproducing part, a head driving mechanism which drives said magnetic head relatively on said magnetic recording medium, and a read and write signal processing means which sends recording signal to said head and processes reproduced signal from said head, in which the reproducing part of said magnetic head is constituted with a magnetoresistive head, and said magnetic recording medium is constituted with the above-mentioned magnetic recording medium of the present invention. Hereby, a small size large capacity magnetic recording system having high recording density and high reliability can be provided.
In relation to the said magnetoresistive head, it is desirable that its magnetoresistive sensor is formed between two shield layers which comprise soft magnetic material and are 0.07 xcexcm or more and 0.2 xcexcm or less apart each other. If the distance between two shield layers is larger than 0.2 xcexcm, sufficient reproduced signal can not be obtained at high linear recording density region over 220 kFCI. If the distance between two shield layers is smaller than 0.07 xcexcm, it comes to be difficult to keep insulation between the shield layer and the magnetoresistive sensor. It is desirable that the product of thickness t of the magnetic layer of said magnetic recording medium by residual magnetic flux density Br measured under a magnetic field given along the running direction of head relatively to the magnetic recording medium at recording; Brxc3x97t is 3 mA (38 gaussxc2x7xcexcm) or more and 7.5 mA (94 gaussxc2x7xcexcm) or less. When Brxc3x97t is smaller than 3 mA, risk of read error is increased, because the reproduced signal is decreased by that recorded bits are left for long time after recording, and when Brxc3x97t is larger than 7.5 mA (94 gaussxc2x7xcexcm), over write at recording is difficult. In the above-mentioned magnetic recording system, as the reproducing part of the magnetic head, it is suitable to employ a magnetoresistive sensor which includes plural conductive magnetic layers in which large resistance variation is occurred by changing respective magnetization direction relatively with external magnetic field, and conductive non-magnetic layers formed between said conductive magnetic layers. By using this head, data signal recorded at high recording density over 300 kFCI can be reproduced stably, and then high recording density over 5 Gbits per square inch can be attained.